1. Field of the Invention
The present invention relates to a method of producing pressure-sensitive copying sheets. In greater detail, it relates to a method of producing pressure sensitive copying sheets by which capsules composing the pressure-sensitive copying sheet are not broken.
2. Description of the Prior Art
Generally, as pressure-sensitive copying sheets, there are known: those comprising a layer of capsules containing a color former coated on one surface of a copying sheet, those comprising a layer of capsules containing a color former coated on one surface of a copying sheet and a layer of a color developer which brings about color development by adsorbing or reacting with said color former on the opposite side of the sheet; those in which layer of capsules containing a color former and a color developer is coated on one surface of the copying sheet, those comprising a multilayer structure of a layer of capsules containing a color former and a layer of a color developer coated on the same side of a copying sheet, and those having a layer of capsules containing a color former on the reverse side of said sheet. (Hereinafter, the term "pressure-sensitive copying sheet" refers to all of these materials.)
Pressure-sensitive copying sheets must possess the property that only in the area in which writing pressure is applied the color intensively and distinctly developed when the desired writing pressure is applied to the pressure-sensitive copying sheet. It has been clarified by Japanese Patent Publication No. 33204/73 corresponding to U.K. Pat. No. 1252858 and U.S. patent application Ser. No. 814,336 filed that the coloring ability of the pressure-sensitive copying sheet is represented by a typewriter intensive index (Ti) defined by the following equation. ##EQU1##
The lower the typewriter intensive index (Ti) is, the higher the coloring ability is.
On the other hand, the pressure-sensitive copying sheets must also have the property that the sheets are difficult to color during handling such that the capsules making up the sheets are not broken thereby. Japanese Patent Publication No. 33204/73 represents this property as a frictional stain intensive index (Fs) defined by the following equation. ##EQU2##
The higher the frictional stain intensive index (Fs) is, the more difficult coloring is in general handling.
The preferred pressure-sensitive copying sheets must possess these two properties. Namely, a low Ti value and a high Fs value.
Particularly, it has recently been required that pressure-sensitive copying sheets be capable of bringing out color in high density with a lower mechanical impact pressure in order to satisfy the demands of high-speed line printers used in electronic computors. Pressure-sensitive recording sheets have also been required to be able to bring out color in high density with low writing pressure in order to make many copies at one time, since it is often necessary to make many copies at one time by pencil, etc.
In order to satisfy the above requirements and to decrease the Ti value, it is necessary to increase the concentration of the color former in the oil contained in the microcapsules in the pressure-sensitive copying sheet or to increase the particle size of the microcapsules or both. However, in such a case, the Fs value becomes low and the capsules break in general handling and stain the background. As a result, it is very difficult to obtain pressure-sensitive copying sheets which satisfy the above described requirements.
By the way, since the Fs value of the pressure-sensitive copying sheets is an indication of the difficulty of coloring in a case that the capsules are broken in handling to cause stains, it has been expected that the pressure-sensitive copying sheet causing less stain and having a low Ti value and a high coloring ability can be obtained even though the Fs value is low, if destruction of the capsules does not occur or occurs to a lesser extent in handling. However, since it has generally been believed that the degree of staining on the pressure-sensitive copying sheets depends on printing machines (see, for example, Japanese Patent Publication No. 33204/73), there is no room for accepting such a way of thinking, and, consequently, it has been believed that the pressure sensitive copying sheets causing less stain and having a high coloring ability are very difficult to produce.